Barker's three maxims for discerning moral truths are what he calls the three moral minds of "instinct, law, & reason".
It seems quite superficial as he does not raise any objections to his proposition. It is actually quite simple to raise objections to this view, that neither instinct, law, nor reason are sufficient in providing a universal moral system.
Instinct is superficial and does not get rid of the idea of the soul entrenched with the Divine conscience that God has given us. Law is temporary & cultural, hence it differs from nation to nation. Reason never works alone and always depends on an arbitrary preference over certain moral actions. It's difficult to separate cognitive bias from reason, as is exemplified by Kant & his deontological morality.
Barker, then, however, goes through various biblical passages that show how immoral God is throughout the Old & New Testament.
He mentions William L. Craig, the Christian Apologist, and shows how weak his arguments are. At times even defending genocide because God ordained it (and that is not straw-manning).
Overall this book is a hard 2.5/5. It's short but doesn't set out to accomplish what it did. It does not sufficiently defend humanist morality but it does do a good job at explaining why the Bible is the quintessence of immorality.